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Well I did not expect THAT!

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Ivan Beaver:
Add this to the "just to be aware of" category of audio oddities

This past weekend I was providing some supplemental subs for a festival in the Miami area. Yes we got wet-thanks Debbie :(

The stage was 40' wide and the main subs were outside that-under the full range hangs.  The subs I brought were in front of the stage in a row, about 25' wide x 45" tall.

We setup on Saturday (show was on Sunday).  Sat night listening and all was well, subs nice and punchy and deep etc.  We were setting up between bands of rain.

I did not pull out my measurement rig as the winds were between 20-30 MPH, so phase be damned at that point.

During soundcheck on Sunday morning one of the guys with the main system provider asked if my subs were in or out of polarity, as the main system subs were OUT of polarity.

Mine were IN polarity.  So I flipped mine (I did not want to mess with the alignment of the main system), and we walked out front and listened and walked a little bit and it was not obvious to us that anything actually changed.  The FOH guy did not make any comment about anything changing for his position.

I thought "That is weird", and went to the model of the system, and flipped the polarity on my subs, and was kind of "dumbfounded" that not a lot changed in the model as well.  The lobes moved around a little and the SPL on some of the mics went up a dB or two and others went down a dB or two, but nothing dramatic.

Then I got to thinking about the actual phase (not polarity) of the different sub signal arrivals at different listening positions, and it started to make sense.

I had never done a setup like that and flipped the polarity of different "groups" of subs.  Yes the polarity of individual subs within the group made big differences in the coverage, but in this particular case, the system could have been run either way and been fine.

In looking at the model (because I can see the coverage much easier than walking around (it was help outside on a huge parking lot-so no reflections to speak of)), when they were all IN polarity, it was a tad bit better coverage wise (which is what I based my model on originally), even though the FOH SPL was down about 2dB (from OUT of polarity).  The SPL loss was not an issue as only on rare instances did we even hit the -6dB (before limit) LEDs on the amps.

Of course with a large scale (meaning many speakers over a wide area) there is no way to get it right for everyone, so you just choose a specific location and hope that is not to bad for other positions.

Just one more thing to add to my "consideration" hat when doing larger shows.

Hey, you learn something every day.  That is a good thing

Riley Casey:
Color me counter intuitive surprised. It'd be interesting to see your in versus out of phase prediction plots.

Keith Broughton:

--- Quote from: Ivan Beaver on August 08, 2024, 09:56:09 AM ---Add this to the "just to be aware of" category of audio oddities


Hey, you learn something every day.  That is a good thing

--- End quote ---
So it's possible the actual phase response of the 2 systems we not the same (say for example 90 deg out ) so when flipping polarity, the results were basically the "same".
Were you able to measure the actual phase angle for each system?

Ivan Beaver:

--- Quote from: Keith Broughton on August 09, 2024, 07:04:24 AM ---So it's possible the actual phase response of the 2 systems we not the same (say for example 90 deg out ) so when flipping polarity, the results were basically the "same".
Were you able to measure the actual phase angle for each system?

--- End quote ---
As I stated, I did not even pull out my measurement rig.

The winds were so strong, there was no chance of getting phase trace to stay still.

Of course the actual phase differences will vary by listener position.

Keith Broughton:

--- Quote from: Ivan Beaver on August 12, 2024, 08:13:01 AM ---As I stated, I did not even pull out my measurement rig.

The winds were so strong, there was no chance of getting phase trace to stay still.

Of course the actual phase differences will vary by listener position.

--- End quote ---
I missed the "too windy" part. Would certainly have made measurement...um... problematic!
The whole sub alignment issue is such a minefield!

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